Definitions and ODR

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

General topics
Preprocessor Comments Keywords	Escape sequences History of C++
Flow control
Conditional execution statements
if statement	switch statement
Iteration statements
for loop range-for loop (C++11)	while loop do-while loop
Jump statements
continue statement break statement	goto statement return statement
Functions
function declaration
lambda function declaration
function template
inline specifier
exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw expression	try-catch block
Namespaces
namespace declaration	namespace aliases
Types
fundamental types compound types enumeration types	union types function types
decltype specifier (C++11)
Specifiers
cv specifiers
storage duration specifiers
constexpr specifier (C++11)
auto specifier (C++11)
alignas specifier (C++11)
Initialization
default initialization value initialization zero initialization copy initialization direct initialization	aggregate initialization list initialization (C++11) constant initialization reference initialization

Literals
integer literals character literals floating-point literals string literals	boolean literals nullptr (C++11) user-defined (C++11)
Expressions
value categories order of evaluation	operators operator precedence
alternative representations
Utilities
Types
typedef declaration
type alias declaration (C++11)
attributes (C++11)
Casts
implicit conversions
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-style and functional cast
Memory allocation
new expression	delete expression
Classes
class declaration this pointer access specifiers	friend specifier initializer lists
Class-specific function properties
virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
default constructor copy constructor move constructor (C++11)	copy assignment move assignment (C++11) destructor
Templates
class template
function template
template specialization
parameter packs (C++11)
Miscellaneous
Inline assembly

 

Definitions are declarations that fully define the entity introduced by the declaration. Every declaration is a definition, except for the following:

1) Any declaration with an extern storage class specifier or with a language linkage specifier (such as extern "C") without an initializer

extern const int a; // declares, but doesn't define a
extern const int b = 1; // defines b

2) Function declaration without a function body

int f(int); // declares, but doesn't define f

3) Parameter declaration in a function declaration that isn't a definition

int f(int x); // declares, but doesn't define f and x
int f(int x) { // defines f and x
     return x+a;
}

4) Declaration of a static data member inside a class definition

struct S {    // defines S
    int n;        // defines S::n
    static int i; // declares, but doesn't define S::i
};
int S::i; // defines S::i

5) Declaration of a class name (by forward declaration or by the use of the elaborated type specifier in another declaration)

struct S; // declares, but doesn't define S
class Y f(class T p); // declares, but doesn't define Y and T (and also f and p)

6) Declaration of a template parameter

template<typename T> // declares, but doesn't define T

7) typedef declaration

typedef S S2; // declares, but doesn't define S2 (S may be incomplete)

• (c++11) alias-declaration

using S2 = S; // declares, but doesn't define S2 (S may be incomplete)

• (c++11) Opaque declaration of an enumeration.

enum Color : int; // declares, but doesn't define Color

• using-declaration

using N::d; // declares, but doesn't define d

• using-directive (does not define any entities)
• (c++11) static_assert declaration (does not define any entities)
• (c++11) Attribute declaration (does not define any entities)
• Empty declaration (does not define any entities)

Where necessary, the compiler may implicitly define the default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, and the destructor.

If the definition of any object results in an object of incomplete type, the program is ill-formed.

One Definition Rule

Only one definition of any variable, function, class type, enumeration type, or template is allowed in any one translation unit.